| Upon this first, and in one sense this sole, rule of reason,
| that in order to learn you must desire to learn, and in so
| desiring not be satisfied with what you already incline to
| think, there follows one corollary which itself deserves
| to be inscribed upon every wall of the city of philosophy:
|
| Do not block the way of inquiry.
|
| Although it is better to be methodical in our investigations,
| and to consider the economics of research, yet there is no
| positive sin against logic in 'trying' any theory which
| may come into our heads, so long as it is adopted in
| such a sense as to permit the investigation to go
| on unimpeded and undiscouraged. On the other hand,
| to set up a philosophy which barricades the road
| of further advance toward the truth is the one
| unpardonable offence in reasoning, as it is
| also the one to which metaphysicians have
| in all ages shown themselves the most
| addicted.
|
| C.S. Peirce, 'Collected Papers', CP 1.135-136.
| From an unpaginated ms. "F. R. L.", circa 1899.
| http://www.princeton.edu/~batke/peirce/frl_99.htm

| Upon this first, and in one sense this sole, rule of reason,
| that in order to learn you must desire to learn, and in so
| desiring not be satisfied with what you already incline to
| think, there follows one corollary which itself deserves
| to be inscribed upon every wall of the city of philosophy:
|
| Do not block the way of inquiry.

The way I read what Peirce is saying here, a "rule of reason" is
just a bit of practical advice for guiding our reasoning process
in such a way that it has a better chance of achieving its goals.
So the "proclamation" part can be any of the usual ways that we
give each other advice, and the "justification" part can be any
of the usual ways that we use to decide whether advice is good.

Incidentally, one of the things that I find most interesting
about Peirce's statement is the way that it makes a connection
between an affective or motivational category, to wit, desire,
and the cognitive category of learning. Not coincidentally,
the same link is made in Aristotle's philosophy, for example,
in the text "On Interpretation" that I cited in this paper: